Resolvers and Encoders – What’s the difference?

Both resolvers and encoders measure the rotary position of a shaft, converting a mechanical position to an electrical signal, but what are the differences between them and which should you use?

Similar to a motor in design with a rotor with a reference winding, a resolver also has a stator with two secondary windings.  These secondary windings, sin and cos are phased 90 degrees mechanically apart.  When you apply an AC voltage to the rotor winding, voltage is induced in the stator winding.

These voltages are equal to the value of the reference voltage multiplied by the sin/cos of the input shafts angle from the zero point.  The ratios of these voltages represent the absolute position of the input shaft, and a Resolver to Digital Converter (RDC) compares the two voltages to determine the shaft position, and gives the result as an analogue signal.

Unlike the mechanical resolver, the encoder is a solid state device producing a digital output.

Better suited to harsh environments, with a higher vibration and shock loading resistance than an encoder.

Resistant to electrical disturbances.

Can withstand a higher temperature than an encoder due to no solid state electronics.

More durable.

Lower weight and rotational inertia than a resolver.

Better suited to applications which have high acceleration and deceleration rates.

Accuracy typically in a range of 20 arcseconds.

Easy to upgrade - often requiring only the feedback element and software to be changed.

Difficult to integrate, the RDC must be tuned to the drive system

More expensive than an encoder

The conversion rate of the RDC limits the maximum working speed.

Loss of accuracy when the application undergoes rapid speed changes

Accuracy within 3 arcminutes

Hard to upgrade, often requires changes to converter, filter, supply voltage and frequency.

Not suited to harsh environments or high temperatures.

Heavy vibration and shock loads can cuase problems.

Not as durable as a resolver